1. Field of the Invention
This invention relates to a semiconductor element, particularly usable for a field-effect transistor (FET), a high electron mobility transistor (HEMT), a heterojunction bipolar transistor (HBT) or the like.
2. Related Art Statement
With the recent development of cellular phone and optical communication techniques, low electric power consumable and high output electronic devices having high frequency properties are remarkably desired. As such an electronic device, conventionally, Si and GaAs devices have been employed. However, since these devices do not have sufficiently high frequency properties, a new high output electronic device is keenly desired.
In this point of view, a HEMT and a psudemorphic HEMT which are made of GaAs-based semiconductors are developed and practically used. Moreover, high performance electronic devices such as a HEMT and a HBT made of InP-based semiconductor have been researched and developed.
However, such a high performance electronic device is composed of plural semiconductor layers epitaxially grown on a given substrate, and has a very complicated structure. Moreover, micro processing techniques are required in fabricating the electron device, so that the manufacturing cost of the electronic device rises. In addition, the InP-based semiconductor is very expensive, so alternatives are desired.
In this point of view, recently, much attention is paid to a new electron device made of a GaN-based semiconductor. Since the bandgap of the GaN semiconductor is 3.39 eV, the GaN semiconductor can have a dielectric breakdown voltage tenfold as large as that of a GaAs semiconductor and a Si semiconductor. Moreover, because the GaN semiconductor can have a large electron saturated drift velocity, it can have a larger performance index as an electronic device than a GaAs semiconductor and a Si semiconductor. Therefore, a GaN semiconductor is prospected as a fundamental semiconductor for high temperature devices, high output devices and high frequency devices in engine controlling, electrical power converting and mobile communication techniques.
Particularly, since an electronic device with an HEMT structure made of an AlGaN or GaN semiconductor is developed by Khan et al. and published in xe2x80x9cJ. Appl. Phys. Lett.,xe2x80x9d 63(1993), pp1214-1215, such an electronic device using GaN-based semiconductor has been intensely researched and developed all over the world. Such an electronic device is generally formed by epitaxially growing given semiconductor layers on a sapphire substrate.
However, since the lattice mismatch between the GaN-based semiconductor layer and the sapphire substrate is large, many misfit dislocations are created at the boundary between the semiconductor layer and substrate, and, propagated in the semiconductor layer. As a result, many dislocations on the order of 1010/cm2 are created in the semiconductor layer, and thus, the electrical properties of the electronic device including the semiconductor layer are deteriorated. Therefore, under the present conditions, the performance of the electronic device made of a GaN-based semiconductor can not be sufficiently improved.
In order to improve the crystal quality of the GaN-based semiconductor layer, an attempt has been made to form a buffer layer between the semiconductor layer and the sapphire substrate or to employ a SiC substrate, a GaN substrate and other oxide substrates instead of the sapphire substrate, but the crystal quality of the GaN-based semiconductor layer formed on the substrate can not be sufficiently improved.
Moreover, an ELO (epitaxial layer overgrowth) technique, such as forming a strip mask made of SiO2, etc. on a substrate has been developed. In this case, misfit dislocations created at the boundary between the GaN-based semiconductor layer and the substrate are laterally propagated in the region above the strip mask, and thus, the dislocation density of the semiconductor layer is decreased in between the adjacent strip portions of the mask.
However, since the ELO technique requires a complicated process, the manufacturing cost of the electronic device rises. Moreover, since the GaN semiconductor layer must be formed thicker so as to cover the strip mask, the substrate may be warped. If the ELO technique is employed in the practical process for manufacturing electron devices including the GaN-based semiconductor layers, most of the substrates to be employed and constituting the electronic devices are warped and thus, broken. Therefore, the ELO technique can not be employed in the practical manufacturing process for the electronic device.
It is an object of the present invention to decrease the dislocation density of a an epitaxially grown semiconductor layer made of a nitride including at least one element selected from the group consisting of Al, Ga, and In, and thus, to provide a semiconductor element including a semiconductor layer usable as a practical device, such as a FET and a HEMT.
In order to achieve the above object, this invention relates to a semiconductor element (a first semiconductor element), substantially including a substrate, an underlayer epitaxially grown on the substrate and made of a first semiconductor nitride including at least elemental Al. The dislocation density of the underlayer is set to 1011/cm2 or below, and the crystallinity of the underlayer is set to 90 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection. A conductive layer is also included, epitaxially grown on the underlayer and made of a second semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the conductive layer is set to 1010/cm2 or below, and the crystallinity of the conductive layer is set to 150 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection.
This invention also relates to a semiconductor element (a second semiconductor element), substantially including a substrate, and an underlayer epitaxially grown on the substrate, made of a first semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the underlayer is set to 1011/cm2 or below, and the crystallinity of the underlayer is set to 90 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection. A carrier moving layer is also included, epitaxially grown on the underlayer, made of a second semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the carrier moving layer is set to 1010/cm2 or below, and the crystallinity of the carrier moving layer is set to 150 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection. Further, a carrier supplying layer is also included, epitaxially grown on the carrier moving layer and made of a third semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In.
Moreover, this invention relates to a semiconductor element (a third semiconductor element), substantially including a substrate, and an underlayer, epitaxially grown on the substrate, made of a first semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the underlayer is set to 1011/cm2 or below, and the crystallinity of the underlayer is set to 90 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection. A first conductive layer of a first conduction type is provided, epitaxially grown on the underlayer and made of a second semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the first conductive layer is set to 1010/cm2 or below, and the crystallinity of the first conductive layer is set to 150 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection. A second conductive layer of the first conduction type is also provided, epitaxially grown on the first conductive layer and made of a third semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the second conductive layer is set to 1010/cm2 or below, and the crystallinity of the second conductive layer is set to 90 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection. A third conductive layer of a second conduction type opposite to the first conduction type is also provided, epitaxially grown on the second conductive layer and made of a fourth semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the third conductive layer is set to 1010/cm2 or below, and the crystallinity of the third conductive layer is set to 150 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection. Further, a fourth conductive layer of the first conduction type is provided, epitaxially grown on the third conductive layer, made of a fifth semiconductor nitride including at least one element selected from the group consisting of Al, Ga and In. The dislocation density of the fourth conductive layer is set to 1010/cm2 or below, and the crystallinity of the fourth conductive layer is set to 150 seconds or below in full width at half maximum of an X-ray rocking curve at (002) reflection.
The inventors have intensely studied epitaxial growing techniques, particularly, epitaxially growing an AlN film on a sapphire substrate, for a long time. In this process, the inventors found out that if the AlN film is epitaxially grown on the sapphire substrate by employing specific conditions, misfit dislocations are entwined at the boundary between the film and the substrate though the misfit dislocations are created at the boundary, and thus, are not propagated in the film.
Therefore, the dislocation density of the AlN film epitaxially grown can be reduced, and thus, the crystal quality of the AlN film can be developed. Such an astonishing phenomena can not be observed under conventional conditions for epitaxially growing AlN films. Concretely, the dislocation density of the AlN film can be reduced to 1011/cm2 or below, and the crystallinity of the AlN film can be enhanced to 90 seconds or below in full width at half maximum (FWHM) of an X-ray rocking curve.
Moreover, the inventors found out that if a GaN film is formed on the AlN film, the dislocations in the AlN film are entwined at the boundary between the AlN film and the GaN film due to the difference in lattice constant there-between, and thus, can not be propagated in the GaN film. Therefore, the dislocation density of the GaN film can be reduced to 1010/cm2 or below, and the crystallinity of the GaN film can be enhanced to 150 seconds or below in FWHM of an X-ray rocking curve.
If the AlN film is employed as an underlayer and a semiconductor film made of a nitride and constituting a conductive layer is epitaxially grown on the underlayer, the crystallinity of the semiconductor layer is improved, originating from the high crystallinity of the underlayer and the dislocation density of the semiconductor layer being improved. As a result, the electrical properties, such carrier mobility, of the conductive layer made of the semiconductor layer can be enhanced.
The semiconductor element of the present invention conceived from the long-term research and development as mentioned above can be employed as a practical device such as a FET, a HEMT and a HBT. Then, it has been expected to employ such a semiconductor element having a semiconductor layer made of a nitride including at least one element selected from the group consisting of Al, Ga and In as a practical device such as a FET, a HEMT and a HBT.
On the other hand, if the above-mentioned ELO technique is employed, a semiconductor layer made of a nitride and having a relatively low dislocation density can be epitaxially grown on a given substrate. Therefore, an underlayer and a conductive layer having a relatively high crystallinity and a relatively low dislocation density can be also fabricated. However, finally, a SiO2 mask remains in the resulting semiconductor element fabricated by using the ELO technique.
In this point of view, in the present invention, the wording xe2x80x9csubstantially comprisingxe2x80x9d means xe2x80x9cnot including such an unnecessary component as a mask in a semiconductor element.xe2x80x9d Since such an unnecessary component is not included in the first through the third semiconductor elements, their semiconductor elements are different from the one fabricated by using the ELO technique.
The first semiconductor element can be preferably employed as a practical device such as a FET, and the second semiconductor element can be preferably employed as a practical device such as a HEMT. The third semiconductor element can be preferably employed as a practical device such as a HBT.